Comparison of Developmental and Stress-Induced Nodule Senescence in Medicago truncatula

Abstract

Mature indeterminate Medicago truncatula nodules are zonated with an apical meristem, an infection zone, a fixation zone with nitrogen-fixing bacteroids, and a “developmental” senescence zone that follows nodule growth with a conical front originating in the center of the fixation zone. In nitrogen-fixing cells, senescence is initiated coincidently with the expression of a family of conserved cysteine proteases that might be involved in the degradation of symbiotic structures. Environmental stress, such as prolonged dark treatment, interferes with nodule functioning and triggers a fast and global nodule senescence. Developmental and dark stress-induced senescence have several different structural and expression features, suggesting at least partly divergent underlying molecular mechanisms.

The Legume Root Nodule: From Symbiotic Nitrogen Fixation to Senescence

“…Ultrastructural analysis of M. truncatula mature nodule cells has revealed at least two stages during the developmental senescence of N 2 -fixing cells: first, a disintegration of bacteroids and symbiosomes revealed by the presence of numerous membranes in the plant cytoplasm associated with the formation of lytic symbiosome compartments probably involved in reabsorption processes and second, the decay of plant infected cells associated with collapse phenomena and that of the plant uninfected cells (Perez Guerra et al, 2010;Timmers et al, 2000;Van de Velde et al, 2006;Vasse et al, 1990). The fusion of symbiosomes to form lytic compartments resembles vacuole formation.…”

“…Thus, SIS is a much faster process than developmental senescence. Moreover, whereas developmental senescence is associated with the establishment of the nodule senescent zone which increases over time, SIS induces the degeneration of the whole nodule in a short time period (Matamoros et al, 1999;Perez Guerra et al, 2010;Vauclare et al, 2010). At the structural level, microscopic analyses also show that developmental senescence and dark stress-induced senescence present different features in M. truncatula.…”

“…At the structural level, microscopic analyses also show that developmental senescence and dark stress-induced senescence present different features in M. truncatula. Dark-induced senescence leads to the condensation of the bacteroid content whereas the PBM remains intact even though most of the bacteroid content had disappeared (Perez Guerra et al, 2010). In contrast, developmental senescence induces a pronounced vesicle mobilisation in the host cytoplasm which is correlated with the degeneration of the PBM and the mixing of the symbiosome content with the cytoplasm (Perez Guerra et al, 2010; Van de Velde et al, 2006).…”

“…The accumulation of catalytic Fe has been detected during dark stress (Gogorcena et al, 1997;Becana & Klucas, 1992) and drought stress (Gogorcena et al, 1995) and participates in OH  production during dark stress (Becana & Klucas, 1992). The modification of iron metabolism during SIS is also noticeable through the up regulation of ferritin and metallothionein during drought stress (Clement et al, 2008) and dark stress (Perez Guerra et al, 2010). These proteins sequester free Fe to decrease the Fenton reaction and protect the cellular primary components.…”

“…At the transcriptomic level, analysis of the expression of 58 genes up regulated during developmental senescence has been performed during dark stress (Perez Guerra et al, 2010). 21 genes are induced during both types of senescence.…”